1. Field of the Invention
The present invention relates generally to a method of processing multi-path signals, and more particularly to a method of processing multi-path signals including a combining operation.
2. Description of the Related Art
Conventional CDMA receivers typically include Rake receivers. Rake receivers ideally are filters matched to the channel impulse response. In practice, the channel impulse response is not known and an estimate for the channel must be used. The estimation of the channel includes estimation errors (e.g., errors in the signal, errors from noise, etc . . . ) which may cause performance degradation.
By conventional methods, the estimation of the frequency selective channel impulse response may be calculated as a superposition of flat fading multi-path signals. The flat fading multi-path signals may correspond to or be assigned to individual fingers of a conventional Rake receiver. The self-interference (i.e., interference to a multi-path signal from another multi-path signal) of multiple propagation paths may be reduced by the despreading operation of conventional CDMA systems.
By conventional methods, each finger of a Rake receiver requires an estimation and compensation of the fading channel response and the multi-path delay. The estimation of the channel may be performed with a low-pass filter and the multi-path delay may be determined with an early-late gate algorithm. In order to compensate for the flat fading channel, the overall delay for all Rake fingers is aligned and the conjugated channel estimates are multiplied with the received Rake finger signals.
Thus, the received signals are time-aligned and co-phased and may be combined with a simple summation; namely, a combining operation or a maximum ratio combining.
However, the maximum ratio combining operation typically assumes perfect channel knowledge. Alternatively, a more general approach employs imperfect channel estimates in the maximum ratio combining operation which includes channel estimation errors.
By conventional methods, when multi-paths are closely spaced, (e.g., spaced less than one chip period apart), the individual multi-paths cannot be resolved and the Rake fingers will lose synchronization with the multi-path signals on the propagation paths. Further, when the multi-path signals are closely spaced the Rake fingers will encounter a superposition of the multi-path signals.